(a) Field of the Invention
The present invention relates to a method and compositions for providing a repulpable moisture vapor barrier coating for flexible packaging as well as to a flexible packaging provided with such coating. More particularly, the invention is concerned with a method for reprocessing a flexible packaging material having a moisture vapor barrier coating thereon, wherein the coating is permitted to be mixed with cellulose fibers to constitute a pulp, allowing the fibers to be adapted to make up a pulp. The invention is also concerned with a flexible packaging which is provided with a coating enabling it to be reprocessed into valuable paper products.
(b) Description of Prior Art
It is current practice to add a film of polyethylene, a wax coating or a polyvinylidene chloride (PVDC) coating on a paper substrate or between two sheets of paper, the paper substrate and the sheets of paper being referred to as liners, in order to obtain a moisture vapor barrier flexible packaging. Presently, all these moisture barrier coatings are considered to be non-repulpable.
Other resins are also used to give flexible packaging materials having a low moisture vapor transmission rate, such as polyacrylates, polyvinyl acetates, and the like. However, for the same barrier performances, they are more expensive than coatings based on wax, polyethylene and polyvinylidene chloride.
Moisture barrier coatings which are present in moisture barrier packaging materials are considered by recycling (repulping) mills to be non-repulpable, mainly because they introduce quality problems in the fiber recovery process, either by upsetting the process (plugging the screen) or by contaminating the finished product.
Presently, more than 20% of all papers and cardboards produced in the world are laminated as indicated above, which give products that are incompatible with the industry of recycling.
One drawback with polyethylene, wax or PVDC coated packaging is that they are difficult to reprocess or recycle and must usually be discarded. The disposal of moisture barrier packaging materials has become an important issue for paper mills and their customers. Repulping (recycling) these materials poses special problems to the industry. The moisture barrier presents a challenge in recovering the useful fiber from these packaging materials, and most recycle mills are unable to overcome the problem of repulsing them. Presently, nearly all of these moisture barrier packaging materials are disposed of in landfills and are not repulpable.
On the other hand, reprocessing of wood fiber based packaging constitutes an important source of wood fibers. Furthermore, because of the above difficulties, these materials must usually be discarded and with recent concerns over environmental problems, this is not acceptable. Reprocessing of wood fiber based packaging is an increasingly important source of wood fibers, and the wastage of high quality and costly fibers is no longer tolerable, since the latter represent millions of tons of waste material.
Two methods are normally used for reprocessing wood fibers. The first method involves the breaking up of the source of wood fibers, such as those present in a packaging material, into constituent fibers as a result of repulping, while any other material is screened away by means of conventional equipment. The second method involves the breaking up of the packaging in such a way that any additional material such as a coating would break up into tiny pieces less than 1.6 mm which would pass through the screen with the fibers to constitute a pulp. This second method is normally carried out with additional equipment and/or chemicals, which makes it quite expensive.
Unfortunately, none of the resins of the prior art with or without wax which are used to provide coatings for flexible packaging can be reprocessed without additional manufacturing steps, with the result that recyclability is difficult. In addition, the presence of wax in the moisture vapor barrier coating lowers the usable pulp yield and therefore increases the amount of waste.
In the repulping process, wax based barriers break up into very tiny particles (less than 0.7 mm) which pass through the screen and end up in the pulp which is sent to the paper machine, as well as in the white water. Problems associated with repulping wax based barriers are the following:
the wax particles plug up the felts; PA1 the wax particles gum up the can dryer causing paper breaking; PA1 wax ends up at the surface of the product being made resulting in surface and printing problems and causing stickies in the finished product; and PA1 lowering of the usable pulp yield.
On the other hand, when polyethylene is used as the vapor barrier coating, while in the repulper, it breaks up into large pieces of film whose sizes range from about 0.3 cm to 2.5 cm long. Polyethylene causes screen plugging, requires downtime to clean and generate solid waste.
The problems associated with polyvinylidene chloride barrier coatings are generally the same as those found when the barrier coatings are made of polyethylene.
Since PVDC has the advantage of providing a coating with excellent vapor barrier, good oxygen barrier, as well as chemical resistance properties at a relatively low cost, it would be commercially beneficial for the manufacture of paper rolls or the like to be able to rely on a flexible packaging including a PVDC coating that can be completely recycled and reprocessed.
However, with the present state of the art, any attempt to modify the repulpability of PVDC and other types of resins, by adding materials or treating the packaging to increase the hardness of the film, would result in a tremendous loss of the barrier properties, which is of course not acceptable.
Coatings based on PVDC are known, for example as taught in UK 1,583,947 published Feb. 4, 1981, inventor Frederic Douglas Hough. However, these coatings are used on paper to produce a transfer sheet. The Patent is mute with respect to the recycling of this coated paper, and gives no directive to produce a coating which can be repulped along with the paper fibers.
UK 2,039,789 published Aug. 10, 1980, inventor Adrian Neville Fellows, describes the preparation of a dielectric coating from a dispersion of an electrically insulating polymer and a water dispersible smectite clay. There is no teaching in this Patent of providing a coating which is characterized by being water vapor impermeable, while being repulpable when broken into a pulp mixture.
Richard M. Podhajny in "The Search for Alternative Moisture-Barrier Coatings", Converting Magazine.RTM., August 1995, suggest that alternatives to PVDC are urgently needed because of the polluting character of PVDC when allowed to be disposed in waste stream. In spite of the fact that PVDC coatings, with or without additives, are known, it will thus appear that a suitable formulation wherein the coating will remain with the pulp when recycling and therefore will not pollute the environment, has not yet been achieved.
At the present rate, it will not be possible to maintain the traditional rate of supplying paper source from forests, and means will have to be found to obtain sufficient fibers to meet the demand.